Veteran FEMA staff members who have surveyed the destruction report some of the worst damage they have ever seen, he said.

"This is truly a catastrophic event," Brown said on CNN's "Larry King Live."

Louisiana and Mississippi officials urged evacuees as well as those stranded by flooding from the storm to stay put.

"It's too dangerous to come home," said Louisiana Gov. Kathleen Blanco, who ordered state police to block re-entry routes to all but emergency workers. (Full story)

A Louisiana public health expert described conditions in New Orleans that are probably typical in all the stricken areas:

"No sewage, no drinking water, contamination, threat of rapid increase in mosquitoes, roads are impassible, downed power lines everywhere, trees, debris from houses in the roads, no way to go shopping, no gas," said Ivor van Heerden, director of the Center for the Study of Public Health Impacts of Hurricanes in Baton Rouge.

More than 75,000 people were being housed in nearly 240 shelters across the region, and "we expect that to grow" as people who can't return home seek somewhere to stay, Evans said.

Baby formula, communications equipment, generators, water and ice are being rushed into hard-hit areas by the federal government, along with doctors, nurses and first-aid supplies, The Associated Press reported.

Experts to help with search-and-rescue efforts have been dispatched by the Pentagon, the AP said.

Wal-Mart said it has donated $1 million to the Salvation Army for disaster relief and that more than 80 stores in the Gulf Coast states would help distribute relief goods.

The home improvement chain Lowe's said it would match customer donations up to $1 million.

T-Mobile said it was offering Wi-Fi Internet service free of charge at its HotSpot locations in Louisiana, Mississippi and Alabama.

The HotSpot service is offered at Borders Books and Music, FedEx Kinko's, Starbucks, Hyatt Hotels, Red Roof Inn Motels, American Airlines, Delta Air Lines and US Airways clubs and lounges.